(1) synapse formation, synapse specificity and neurotransmission were studied using neurons and muscle cells in culture. Synapses were detected and investigated by electrophysiological recording. Neurons from chick and rat retina and chick spinal cord form synapses with muscle cells. Each type of neuron possesses a synapse-competent state in its development during which it can abundantly form synapses with muscle cells in culture. In addition, developmental periods for maximal neurite extension correlate with the developmental periods for synaptogensis. These results suggest a mechanism of synapse specificity based on a synaptogenic period during development for each type of neuron. Synapse specificity can also be based on differences in synapse stabilization. Inappropriate synapses, such as those from rat or chick retina neurons on muscle cells, are transient; whereas appropriate synapses, such as those from spinal cord on muscle, are stabilized. (2) Neuroblastoma cell lines can be used as in vitro model systems for neural function. Three neuroblastoma cell lines have been characterized for their morphology, neurochemistry and ability to form synapses. (3) Neurotransmitter release can be studied in an in vitro perfusion system. Acetylcholine release increases during development and maturation of chick retina synapses. Dopamine release from rat striatal slices does not change during aging.